Mothers who bear their first child by their late teens have about half the risk of developing breast cancer relative to nulliparous women. The rat is a good model for studying the role of hormones in breast cancer since, for example, young rats become nearly refractory to mammary carcinogenesis after delivering a litter of pups. Administration of high doses of estradiol and progesterone to rats can yield serum titers of these steroids which approximate their levels reached during pregnancy. When virgin rats are treated with high doses of estradiol and progesterone, they become as refractory to chemically-induced mammary carcinogenesis as parous rats with no apparent toxic effects. Our objective is to understand the mechanism by which high doses of these two hormones protect the mammary gland from developing cancers. Our hypothesis is that treatment with estradiol and progesterone differentiates transformed mammary epithelial cells, thereby preventing them from developing into frank carcinomas. We propose to use morphological, histological, biochemical and molecular methods to study the biology of the carcinogen-exposed mammary gland as a function of estradiol and progesterone treatment. We will also test an alternative hypothesis that these hormones act systemically rather than directly on the mammary epithelial cells. Mammary epithelial cells from hormone treated or control donors will be transplanted into treated or control recipients to determine if the refractory phenotype segregates with treatment of the mammary cells or treatment of the rats in which the cells are transplanted. We also plan a series of studies to determine the minimum dose and length of hormone treatment necessary to confer protection of the mammary gland from carcinogenesis. These studies should provide important information regarding the mechanism by which high levels of estrogen and progesterone protect the mammary gland from carcinogenesis and may help to provide the basis for fixture chemoprevention trials in women.